/* 
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

using System;
using Lucene.Net.Support;
using IndexReader = Lucene.Net.Index.IndexReader;

namespace Lucene.Net.Util
{
    /// <summary>
    /// Common util methods for dealing with <see cref="IndexReader" />s.
    /// </summary>
    public class ReaderUtil
    {
        /// <summary>Gathers sub-readers from reader into a List.</summary>
        /// <param name="allSubReaders"></param>
        /// <param name="reader"></param>
        public static void GatherSubReaders(System.Collections.Generic.IList<IndexReader> allSubReaders, IndexReader reader)
        {
            IndexReader[] subReaders = reader.GetSequentialSubReaders();
            if (subReaders == null)
            {
                // Add the reader itself, and do not recurse
                allSubReaders.Add(reader);
            }
            else
            {
                for (int i = 0; i < subReaders.Length; i++)
                {
                    GatherSubReaders(allSubReaders, subReaders[i]);
                }
            }
        }
        
        /// <summary> Returns sub IndexReader that contains the given document id.
        /// 
        /// </summary>
        /// <param name="doc">id of document
        /// </param>
        /// <param name="reader">parent reader
        /// </param>
        /// <returns> sub reader of parent which contains the specified doc id
        /// </returns>
        public static IndexReader SubReader(int doc, IndexReader reader)
        {
            var subReadersList = new System.Collections.Generic.List<IndexReader>();
            ReaderUtil.GatherSubReaders(subReadersList, reader);
            IndexReader[] subReaders = subReadersList.ToArray();
            int[] docStarts = new int[subReaders.Length];
            int maxDoc = 0;
            for (int i = 0; i < subReaders.Length; i++)
            {
                docStarts[i] = maxDoc;
                maxDoc += subReaders[i].MaxDoc;
            }
            return subReaders[ReaderUtil.SubIndex(doc, docStarts)];
        }
        
        /// <summary> Returns sub-reader subIndex from reader.
        /// 
        /// </summary>
        /// <param name="reader">parent reader
        /// </param>
        /// <param name="subIndex">index of desired sub reader
        /// </param>
        /// <returns> the subreader at subINdex
        /// </returns>
        public static IndexReader SubReader(IndexReader reader, int subIndex)
        {
            var subReadersList = new System.Collections.Generic.List<IndexReader>();
            ReaderUtil.GatherSubReaders(subReadersList, reader);
            IndexReader[] subReaders = subReadersList.ToArray();
            return subReaders[subIndex];
        }
        
        
        /// <summary> Returns index of the searcher/reader for document <c>n</c> in the
        /// array used to construct this searcher/reader.
        /// </summary>
        public static int SubIndex(int n, int[] docStarts)
        {
            // find
            // searcher/reader for doc n:
            int size = docStarts.Length;
            int lo = 0; // search starts array
            int hi = size - 1; // for first element less than n, return its index
            while (hi >= lo)
            {
                int mid = Number.URShift((lo + hi), 1);
                int midValue = docStarts[mid];
                if (n < midValue)
                    hi = mid - 1;
                else if (n > midValue)
                    lo = mid + 1;
                else
                {
                    // found a match
                    while (mid + 1 < size && docStarts[mid + 1] == midValue)
                    {
                        mid++; // scan to last match
                    }
                    return mid;
                }
            }
            return hi;
        }
    }
}